No. That can't be changed to default for now. Vivaldi follows the old Opera tradition of opening them in the foreground. However, you can middle-click, ctrl-click, use the mouse gesture down-up on the link, or right-click and select open in background. Even after two years of not using Olde Opera, it still pissed me off that every time I wanted to "open in new tab," I had to then hunt down the background tab and open it. Here, at least, one has a choice. You can open it in the foreground, which no other Chromium-based browser even offers as an option, or there are four ways to open it in the background. At some point, you may get the option to change the default behavior.